Non-invasive imaging of cell migration, trafficking, and homing is an emerging new field that can provide us with a deeper insight into the dynamics of cell-tissue interactions, as well as provide guidance for the development of novel therapies using stem cells. There is little question that the field of cellular therapeutics will eventually become important to treat, or possibly cure, a variety of neurodegenerative diseases. Thus, further development of sensitive, non-invasive imaging techniques that can be applied clinically is clearly warranted. We propose to develop magnetic particle imaging (MPI) as a novel technique for imaging of stem cells. This imaging modality is based on the non-linear magnetization curve of small superparamagnetic tracers already used with MRI cell tracking, but the technique itself is not related to MRI. In principle, MPI has several advantages over MRI: a) a 1000-fold higher sensitivity;b) the ability to absolutely quantify the amount of magnetic tracers and cells;c) "hot spot" interpretation without the confounding endogenous background signal present in MRI;and d) the absence of obscuring contrast from hemorrhage or traumatic injury. We have preliminary data that magnetically labeled cells can be detected in vitro at low concentrations with a prototype MPI instrument, and that there is a straightforward quantification of cell number and iron content. Following the optimization of labeling procedures with different particles, MPI will be further investigated in vivo in a rat model of focal transient ischemia using middle cerebral artery occlusion. Different doses of mesenchymal and neural stem cells will be infused either intra-arterially or intravenously. The evolution of ischemic stroke will be monitored with various MRI techniques, including perfusion imaging, diffusion weighted imaging, and pH imaging. The therapeutic efficacy of administered stem cells will be assessed using a four-tier behavioral scoring system. The total amount of targeted and localized cells in the ischemic lesion, as determined by MPI, will be correlated with lesion volume and behavioral scores. While this proposal may be viewed as high risk, we believe that all necessary components are in place in order to successfully develop cellular MPI for stem cell therapy of stroke. By using MPI to obtain a better insight in the dynamic processes that govern the in vivo (selective) homing and (non-selective) trapping of cells in the brain and other tissues, we aim to optimize the source of stem cells, administration route, and dose of cell injection, which may ultimately enhance the therapeutic efficacy of stem cell treatment in stroke patients. PUBLIC HEALTH RELEVANCE: Stem cells have the potential to ameliorate or perhaps even cure neurodegenerative diseases such as stroke. We aim to develop a new imaging technique that uses magnetic particles for visualizing transplanted stem cells in the brain. If this new technique is developed successfully, it will help us to understand much better where the cells exactly go into the brain and elsewhere in the body, which will facilitate introducing these stem cells into patients.